The manufacture of optical waveguide fibers has long passed from an early, primarily experimental stage to a fully commercial stage in which a growing number of customer's transmission needs are being satisfied over short and long distances and at various wavelengths corresponding to visible as well as to invisible radiation. The manufacture of commercial fiber typically is based on silica glass technology and involves drawing from a massive body or preform having a cross-sectional refractive index profile as designed for effective guiding of one or several radiation modes.
With respect to most currently used optical fiber, optical waveguide structure can be described in terms of a higher-index core portion which is surrounded by a lower-index portion such as typically, a glass cladding. At the core-cladding interface there may be a relatively abrupt change in refractive index; alternatively, and especially in the case of fibers designed for the transmission of a plurality of modes, refractive index may decrease gradually towards a fiber surface. A refractive index difference between core and cladding typically results from the addition of one or several suitably chosen dopants or additives to otherwise essentially pure silica; e.g., the addition of boron or fluorine results in a lowered refractive index, and the addition of aluminum, germanium, phosphorus, or titanium produces an increased refractive index.
Considerable progress has been made in the development of methods for the manufacture of optical waveguide fiber preforms, and a number of such methods have been found capable of producing preforms from which low-loss fibers can be drawn. One such method is described in U.S. Pat. No. 4,217,027, issued Aug. 12, 1980 to J.B. MacChesney et al. Still, and such progress notwithstanding, development efforts continue towards further reducing optical loss, such reductions being in the interest of lengthening the distance over which signals can be transmitted without amplification or regeneration.
Optical fiber for long distance communications has reached a remarkable state of perfection. For instance, single mode fibers having loss of about 0.20 dB/km are routinely being produced. Nevertheless, there is still great interest in further reducing the signal loss, since even a reduction as small as 0.01 dB/km can translate into a significant increase in the permitted distance between repeaters. This in turn can translate into a significant difference in system cost, especially for transmission systems such as transoceanic fiber optic systems that by necessity have to employ highly complex and thus costly repeaters.
Optical fibers from a silica glass composition have become preeminent in the communication field because advantages such as cost and relative facile processing. However, optical fibers from a silica composition have certain shortcomings which impose limitations on the use thereof. For instance, silica-based optical fibers can transmit light over the limited range of about 0.2-2.0 (um) microns wavelength and have the lowest optical loss of about 0.2dB/km at 1.55 .mu.m wavelength.
In her Ph.D thesis, Rutgers University, 1991, A.E. Miller discloses glass composition of germanium oxide, barium oxide, and gallium oxide.